In the conscious rat, 1-isoproterenol (0.3mg/kg s.c.) decreased about 50% both the glomerular filtration rate (GFR) and the renal plasma flow (RPF), as estimated by the total body clearance of 3H-methoxyinulin and 14C-tetraethyl ammonium. This reduction in GFR and RPF may be the mechanism responsible for the potentiation of toxicity of paraquat and other compounds which are rapidly excreted by the kidney in an unchanged form. Pretreatment with the beta adrenergic blocking agent, propranolol, prevented the decrease by isoproterenol in GFR and RPF. These effects of isoproterenol were not mediated by release of antidiuretic hormone from the posterior pituitary since they were as great in hypophysectiomized as in sham-operated rats. Peak plasma concentrations of isoproterenol after subcutaneous injection (0.3 mg/kg) in intact male Sprague-Dawley rats were about 40 ng/ml.